﻿using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ProjectEulerSolutions.Problems
{
    /*
     * Let r be the remainder when (a−1)n + (a+1)n is divided by a2.

For example, if a = 7 and n = 3, then r = 42: 63 + 83 = 728 ≡ 42 mod 49. And as n varies, so too will r, but for a = 7 it turns out that rmax = 42.

For 3 ≤ a ≤ 1000, find ∑ rmax.

     * */
    class Problem120 : IProblem
    {
        public string Calculate()
        {
            long sum = 0;

            for (int a = 3; a <= 1000; a++)
                sum += (a % 2 == 0) ? (a * a - 2 * a) : (a * a - a);

            return sum.ToString();
        }
    }
}
